Built-in Functions

Call functions inside {{...}} expressions. Arguments can be literal strings or variable names — Jake resolves variable names first, then passes the value to the function.

src = "src/components/Button.tsx"

task info:
    echo "{{basename(src)}}"         # uses variable → Button.tsx
    echo "{{basename(lib/util.ts)}}" # literal       → util.ts

String Functions

Function	Description	Example
uppercase(s)	Convert to uppercase	{{uppercase(hello)}} → HELLO
lowercase(s)	Convert to lowercase	{{lowercase(HELLO)}} → hello
trim(s)	Remove whitespace	{{trim( hello )}} → hello

Path Functions

Function	Description	Example
dirname(p)	Get directory part	{{dirname(/a/b/c.txt)}} → /a/b
basename(p)	Get filename part	{{basename(/a/b/c.txt)}} → c.txt
extension(p)	Get file extension	{{extension(file.txt)}} → .txt
without_extension(p)	Remove last extension	{{without_extension(file.tar.gz)}} → file.tar
without_extensions(p)	Remove ALL extensions	{{without_extensions(file.tar.gz)}} → file
absolute_path(p)	Get absolute path	{{absolute_path(./src)}} → /home/user/project/src

System Functions

Function	Description	Example
home()	User home directory	{{home()}} → /Users/alice or C:\Users\alice
local_bin(name)	Path to ~/.local/bin binary	{{local_bin(jake)}} → /Users/alice/.local/bin/jake
shell_config()	Current shell’s config file	{{shell_config()}} → /Users/alice/.zshrc
launch(target)	Platform command to open a file or URL	{{launch(https://example.com)}} → open https://example.com

launch(target)

launch() returns the platform-specific command for opening a file or URL in the default application. It doesn’t open the target directly — it expands to the correct command, which then runs as part of the recipe.

Platform	Expands to
macOS	open {target}
Linux	xdg-open {target}
Windows	cmd /c start "" {target}

docs_url = "https://jake.helge.dev"

task open-docs:
    {{launch(docs_url)}}

task open-report:
    {{launch(coverage/index.html)}}

On unsupported platforms (anything other than macOS, Linux, or Windows), launch() produces an error at evaluation time and the recipe will not run.

Shell Config Detection

shell_config() detects your shell from $SHELL. On Windows, it understands Windows-style shell paths and falls back to PowerShell or COMSPEC when needed.

Shell	Config File
bash	~/.bashrc
zsh	~/.zshrc
fish	~/.config/fish/config.fish
sh	~/.profile
ksh	~/.kshrc
csh / tcsh	~/.cshrc / ~/.tcshrc
powershell	~/Documents/WindowsPowerShell/Microsoft.PowerShell_profile.ps1
pwsh	~/Documents/PowerShell/Microsoft.PowerShell_profile.ps1

shell_config() returns nothing for plain cmd.exe. home() resolves from HOME on Unix and from HOME, USERPROFILE, or HOMEDRIVE + HOMEPATH on Windows.

Chaining Functions

Functions can be nested — the inner call resolves first:

src = "src/main.ts"

task compile:
    tsc --outFile {{without_extension(basename(src))}}.js {{src}}

Expands to: tsc --outFile main.js src/main.ts

Another example — deploying to a path based on the source filename:

component = "src/components/Button.tsx"

task deploy-component:
    cp {{component}} dist/{{lowercase(without_extensions(basename(component)))}}.js

Expands to: cp src/components/Button.tsx dist/button.js

Error Behavior

If a function receives an argument it can’t handle (e.g., launch() on an unsupported OS), Jake reports the error and stops before executing any commands in the recipe. The error identifies the failing expression.

If a variable name is used as an argument but the variable isn’t defined, Jake expands it as an empty string. basename("") returns "", dirname("") returns ".", and so on — no error is produced. If that silent empty-string behavior would cause problems for your use case, set the variable to a sentinel value and check it with @require.